Conventional component-mounted structures are composed of electronic components such as a semiconductor chip, and a substrate such as a printed circuit board. The electronic component and the circuit board are electrically and mechanically connected together by solder-joining their electrodes to each other. However, the purpose of solder joining is mainly to achieve electrical connection between the electrodes, and the mechanical connection strength therebetween is lower than that obtained by, for example, welding. Therefore, conventionally, an adhesive containing a thermosetting resin is supplied between the electronic component and the circuit board to form a resin-reinforcing portion, thereby to reinforce the solder joint portion.
In such a case, a paste prepared by mixing a solder powder with the adhesive (hereinafter referred to as “solder-resin mixture”) is sometimes supplied between the corresponding electrodes of the electronic component and the circuit board (see, e.g., Patent Literature 1). Thus, the solder-joining of the electrodes and the resin-reinforcing of the solder joint portion using the adhesive can be simultaneously performed. In Patent Literature 1, the melting temperature of the solder powder is set lower than the glass transition temperature of the thermosetting resin, thereby to suppress the thermal stress acting on the joint interface between the resin-reinforcing portion and the electronic component or the circuit board.
However, when a solder having a comparatively low melting point is used to join the electrode terminals as in Patent Literature 1, if a reheating process such as re-flowing is further performed after the electronic component is joined to the circuit board, the solder joint portion will easily melt again. This may reduce the connection reliability between the electronic component and the circuit board.
To avoid this, Patent Literature 2 proposes to add metal particles with high melting point, such as Cu particles, to the solder-resin mixture, to form an intermetallic compound of solder and Cu, and thereby to raise the re-melting temperature of the solder joint portion.